2,4-disubstituted-1,5-pentanediols have been explored for use as flexibilizing glycols in polyester and polyurethane applications. Their preparation has been disclosed previously in U.S. Pat. No. 3,046,311 (Milligan). Milligan initially prepared 2,4-disubstituted-crotonaldehydes by self-aldol condensation of simple aldehydes. For example, self-aldol condensation of n-butyraldehyde gives 2-ethyl-2-hexenal (2,4-diethylcrotonaldehyde) which is used primarily as the precursor to the plasticizer alcohol, 2-ethyl-1-hexanol. These 2,4-disubstituted-crotonaldehydes are in turn condensed with formaldehyde followed by hydrogenation of the resultant 2,4-disubstituted-4-methylol crotonaldehydes to form the 2,4-disubstituted-1,5-pentanediols. However, nuclear magnetic resonance spectroscopy confirms the coproduction of 2,2-disubstituted-1,3-propanediols of the same formulas as structural isomers of the pentanediols. Moreover, these isomers are inseparable by the usual physical methods (fractional crystallization, distillation and chromotography). The quantity of the undesirable propanediols may vary from 5 to 25 percent depending upon the original aldol condensation reaction conditions. This variability in the quantity of the inseparable isomers results in an unacceptable variability in the properties of the finished glycol.
The results of Milligan also indicated a yield of 15 to 63 percent of the 2,4-disubstituted-1,5-pentanediols depending upon the starting crotonaldehyde and even lower yields based on formaldehyde. These low yields are commercially undesirable.
Accordingly, it was undertaken to develop a method of synthesizing 2,4-disubstituted-1,5-pentanediols which produced better yields than the Milligan process and also reduced the amount of undesirable structural isomers coproduced by the reaction. Exploration of different reaction conditions using the catalysts of Milligan showed no improvement in product purities or yield.
Further investigation revealed the possibility of eliminating a reaction step from the Milligan process to thereby improve product purity and yield. Since the reaction of the dialkylcrotonaldehydes with formaldehyde and the reaction of simple aldehydes to form the dialkylcrotonaldehydes are both aldol condensations, the possibility existed to combine these two reaction steps into a single step by condensing two moles of simple aldehyde with one mole of formaldehyde to form the precursor to the 2,4-disubstituted-1,5-pentanediols.
This concept did not initially lead to a useful process since the major product was still the 2,4-disubstituted crotonaldehyde after reaction in the presence of caustic catalyst under the usual aldol condensation conditions. Milder aldol condensation catalysts such as triethylamine showed little improvement. However, the employment of secondary amines and their acid salts led to clearly superior results.
These secondary amine catalysts and their acid salts generally fall into the category of enamine catalysts or Mannich catalysts. Enamine or Mannich catalysts are known to aid the condensation of simple aldehydes with formaldehyde to yield alpha-methylol aldehydes or, upon dehydration, alpha-methylene aldehydes (alpha-alkyl acrolein). Farberov, Mironov, and Korshunov, Chem. Abs., 59, 394 (1963), for example, details the high yield Mannich reaction of butyraldehyde with formaldehyde to form alpha-ethyl acrolein by using stoichiometric quantities of dimethylaminehydrochloride as the Mannich reagent. However, this reaction does not produce the 2,4-disubstituted glutaraldehydes needed as an intermediate for the production of 2,4-disubstituted-1,5-pentanediols.
Accordingly, it is the primary object of the present invention to provide a process for preparing 2,4-disubstituted-1,5-pentanediols of higher purity and increased yields.
It is a further object of the present invention to provide a process for the direct preparation of 2,4-disubstituted glutaraldehydes from formaldehyde and simple aldehyde.
These and other objects of the present invention will be apparent to one of ordinary skill in the art from the summary and detailed description which follow.